Platform-type wheelchair lifts of the type disclosed in U.S. Pat. No. 4,058,228 to Hall have been used extensively in passenger vehicles, particularly urban buses. Platform lifts of the type disclosed in the Hall patent are typically installed in one of the existing stairwells in a vehicle. These lifts generally comprise a platform which is slidably mounted in the vehicle beneath the stairwell so as to define the bottom step of the stairwell when in the retracted position and so as to provide a platform which projects outwardly from the stairwell when in the extended position. The platform is attached via a parallelogram linkage assembly to a carriage. The latter is slidably mounted in a pair of opposing channel members and is caused to move between retracted and extended positions by a chain drive assembly which is actuated by a hydraulic or pneumatic linear actuator. The parallelogram linkage assembly is designed to cause the platform to move between upper and lower positions relative to the carriage when the platform is in the extended position. The parallelogram linkage includes two pairs of parallel linkage arms, each arm having a proximal end which is pivotally mounted to the carriage and a distal end which is pivotally mounted to the platform.
The linkage arms are caused to pivot about their proximal ends, thereby causing the platform to move between upper and lower positions, by a sprocket and chain drive assembly. This assembly includes a toothed driving sprocket coupled with the proximal end of one of the parallel arms so as to rotate with the arm and a driven toothed sprocket attached to the proximal end of the other arm so as to rotate with the arm. Rotational drive is transmitted from the driving sprocket to the driven sprocket by a chain drive assembly comprising a single length of leaf chain. One end of the leaf chain is wrapped partially around and is attached to the driving sprocket and the other end of the leaf chain is wrapped partially around and is attached to the driven sprocket. The driving sprocket is driven by a separate chain and sprocket drive assembly which is rotated by a actuator mechanism. In an alternative embodiment of the above-described chain drive assembly, two lengths of roller chain are employed, each of which is wrapped around a respective one of the sprockets. Two turnbuckles are provided for coupling the ends of the roller chains together so as to form a continuous flexible drive member. A preload can easily be applied to chain assembly of the alternative embodiment by appropriate adjustment of the turnbuckles.
Platform lifts of the type described above have been used extensively in urban buses with very favorable results. Recently, the need has arisen (a) to increase the length of the parallel linkage arms of known platform lifts, (b) to increase the size, and hence weight, of the platform of known platform lifts, and (c) to accommodate heavier loads on known platform lifts. However, due to space limitations inherent in the design of the above-described platform lifts, it has not been possible to accommodate A roller chain in the chain and sprocket drive assembly of such lifts of a size, and hence strength, sufficient to permit the lift to be modified in the manner described in the preceding sentence.
Under conventional operation, the parallel arms of platform lifts of the type disclosed in U.S. Pat. No. 4,058,228 are caused to travel between a downwardly projecting position, through a horizontal position, to an upwardly projecting position. As the arms travel through the horizontal position, the tendency exists for the platform of the lift to "buck" or bounce. Such "bucking" is believed to occur due to stretching of the inherently flexible roller chains used in the chain and sprocket drive assembly which arises when the load applied to the roller chains is shifted from the linkage assembly to the chains as the parallel arms move from the upwardly projecting to the downwardly projecting positions, or vice versa. As those who have experienced anomalous vibration or bouncing of elevators can appreciate, such "bucking" of the platform lift can be very disconcerting to a wheelchair occupant positioned on the lift.
In an attempt to minimize the "bucking" of the platform of conventional platform lifts which occurs as the parallel arms travel through the horizontal position, a significant preload has been applied to the roller chains. Although such preloading does reduce the tendency of the platform to "buck," it simultaneously increases the load on the bearings and the strain on the chain, sprockets, and other components of the chain and sprocket drive assembly. Also, the friction between movable components of the chain and sprocket drive assembly is increased as a consequence of such preloading. Such increases in load, strain, and friction result in added maintenance costs and reduced product longevity.
Moreover, many platform lifts are designed so that gravity provides the force needed to move the platforms from the "floor" position adjacent the floor of the vehicle to either the stowed position or the full deployed, ground, position. Problems arise when a platform is in the floor position, because the linkage arms may be at or near a top-center relative to their pivot shafts. When the linkage arms approach this position, gravity may not be able to immediately overcome friction forces that hold the arms in place. As a result, when the linkage arm actuating mechanism that controls the movement of a platform is released, the arms may momentarily be frozen until gravity overcomes the friction forces so as to move the lift downwards. The platform will then momentarily move downwards in a free fall until the actuating mechanism catches the linkage arms so as to control the rate at which the platform descends. As a result, the platform may be subjected to a jolting movement which unnecessarily strains the components forming the lift and which a passenger may find disconcerting.
In order to provide a "push-off" force for initially moving the platform away form the associated vehicle floor and the linkage arms away from their top-center positions, many lifts are provided with a spring-loaded torque shaft assembly. This assembly includes a torque shaft that extends between the two opposed driving sprockets and a torsion spring that extends between the bar and adjacent fixed portion of the frame of the lift assembly. As the platform moves upwards to the floor position, the spring is wound up by the rotation of the torque shaft. When it is time to lower the platform, the spring is allowed to unwind; this motion imparts the necessary push-off force to the linkage arms so that they move past their top-center lines. Gravity is then able to move the platform downward to the desired lowered position.
A disadvantage of spring-loaded torque shaft assemblies is that in order to install such an assembly, the torque shaft must be formed out of two separate rods that are linked together by a coupling member. The expense of providing a three-piece torque shaft adds to the overall cost of manufacturing and installing the platform lift. Furthermore, a three-piece torque shaft is inherently weaker and less able to transfer rotational power than a one-piece shaft of comparable size. Also, the springs used in these assemblies are inherently weak, and given that they are exposed to extreme and repetitive stress, they are prone to break. Still another disadvantage of these spring-loaded torque shaft assemblies is that the torsion springs are located near the center of the lift assemblies in which they are installed. This makes replacing the springs a complicated and time-consuming task.